This invention relates to a device for driving four wheels of a car and more particularly to a device for driving four wheels at all times, suitable for use in passenger cars.
It has heretofore been customary for a long period of time that the so-called selective system is mainly used in four-wheel passenger cars wherein the driver operates to switch a rigid 4 WD for direct connection between the front and rear steel shafts and 2 WD for driving only one of the wheel shafts by releasing the aforesaid connection.
However, along with the fact that the merit of four-wheel drive on the usual road has been recognized as important, the drawbacks of the selective system have become also recognizable. Such drawbacks are concerned with the problem of tight-turn braking. Namely at the time of turning the car, the front wheels and rear wheels depict circles of different radii, thus causing different distances of their movements so that the difference in rotation of the wheels cannot be avoided. In order to avoid such inconvenience, it becomes necessary to incorporate differential gears between the front and rear wheel shafts. In this way, shifting operation of 2 WD and 4 WD may be dispensed with. However, in the case of differential gears consisting of usual bevel gears or planetary gears. the structure concerned is such as to delay the rotation of the resistance-loaded shaft so that when counter force on one-side output shaft becomes less, the shaft will rotate idly until relative driving force is lost. Therefore, in order to prevent the aforesaid condition assuredly in the case of the center-differential 4 WD, such a mechanism becomes necessary as a differential lock or limited slip system. Yet, the center-differential 4 WD becomes complicated in construction and moreover, there occurs the drawback of a braking phenomenon at the time of locking. On the other hand, there is a viscous coupling structure for transmitting torque by utilizing the shearing resistance of viscous fluid (silicon oil).
In this case, although the viscous coupling can transmit torque when there is a difference in rotary speed while the input shaft and output shaft are rotating in an integral manner, yet the size of torque is changed according to a change in rotary speed, namely according to the speed at which the plate shears silicon oil. However, there occurs energy loss due to an exothermic condition on one hand and on the other hand, after the idle rotation or stoppage of one wheel alone has occurred, the other wheels initiate an increase in speed so that the drawback of the time lag is raised. Still as another process, the Torsen center-differential system may be available. Also in this case, there occurs such drawbacks as a tendency of rotary lowering due to the use of worm gears and overload of torque distribution on the right and left rear wheels or front wheels at the time of idle rotation. Even in the case of adopting any one of the aforesaid processes, the idle rotation or stoppage of one wheel only can not be prevented nor can be a proper distribution of torque be ensured after all.
All the aforegoing conditions stand much in the way of propagation of 4 WD cars because of complicated construction and high cost.